1 List of United States cities by population. ^ 2  Population projection is that of the entity known since 2016 as Ciudad de MÃ©xico and previously as the Distrito Federal. ^  Population estimate is that of the Toronto census division, which consists solely of the City of Toronto.
Lens (optics) For other uses, see Lens. A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis.
An important part of an opticianâs job is to make sure that customers are comfortable enough with their new eyeglass frames (or contact lenses) to wear them as prescribed.uring consultations, an optician considers the type of lenses as well as the context in which a customer will use them (for example at a construction site versus an office cubicle) to make a recommendation well suited to the individual's lifestyle.